Blade Mounting

ABSTRACT

The angular nature of corners  3  to roots  2  of a conventional blade  1  used in a gas turbine engine has resulted in relatively thick fan casings in order to contain any blade fragments from these corners  3  as a result of blade failure. By angling the end face  26  of the root  22  such that it is nearer to perpendicular to an axis of curvature  27  for a blade  21  the corners  23  are reduced in their angular nature. In such circumstances the potential for penetration of a fan casing through impingement with such corners  23  is reduced and the fan casing thickness can more confidently be thinned reducing overall weight. Furthermore by appropriate profiling of an aerofoil  24  adjacent to the front and trailing edges  20  of the aerofoil about the root  22  and face  26  further reductions in angular parts of the blade can be achieved.

The present invention relates to blade mountings and blades utilised ingas turbine engines.

Operation of gas turbine engines is relatively well known and includes anumber of aerofoil blades secured in mountings in different stages ofthe gas turbine engine. These blades are generally secured through rootmountings which may take the form of dovetail root sections which entera reciprocally shaped slot in order to secure the blade to a rotor disc.Normally the airfoils forming the blade are curved. In suchcircumstances front and rear edges of the blade root are cut to providean orthogonal flat face for consistency with the rotor disc edgesurfaces at the front and rear edges of the blade. Thus, these bladeroot edges include relatively sharp corners and angular parts.

It will be understood that blades within a gas turbine engine rotate atrelatively high speeds. In such circumstances it is possible for theseblades to fail and therefore sections of the blade to be projected withsome force upon disintegration of the blade. Angular and pointed partsmay exacerbate impact problems.

The acute corners if they impact against a casing when a fan blade failscan cause problems. The acute corners concentrate impact load from therelatively heavy root section of the blade upon disintegration. In orderto prevent penetration though the engine casing it will generally bethicker in cross section to ensure that the blade will not puncture andpass through the casing. Clearly, additional thickness to a fan casingadds considerably to the necessary weight of the fan casing withdetrimental effects upon engine operational efficiency. It is found thata 1 mm increase in thickness in a large fan casing can add approximately16 kg to overall weight.

In accordance with a, first aspect of the invention, there is provided ablade for a gas turbine engine, the blade rotatable in use about an axisof rotation, the blade comprising a root for securing the blade, theroot curved for alignment in use with a mounting slot, characterised inthat an end of the root is substantially perpendicular to the axis ofcurvature of the root.

Alternatively, both ends of the root may be substantially perpendicularto the axis of curvature of the root.

According to a second aspect of the invention, there is provided a bladefor a gas turbine engine, the blade rotatable in use about an axis ofrotation, the blade comprising a root for securing the blade, the rootcurved for alignment in use with a mounting slot, characterised in thatan end of the root makes an angle of substantially 25 degrees to theplane perpendicular to the axis of rotation.

Alternatively, both ends of the root may make an angle of substantially25 degrees to the plane perpendicular to the axis of rotation.

In either the first or the second aspect of the invention, the blade maycomprise an aerofoil which extends from the root and a portion of theaerofoil adjacent to the end of the root may be rounded so as to providea smooth transition between the aerofoil and the end of the root.

At least one end of the root may be chamfered or truncated, so as toreduce the axial length of the root.

The blade may be part of a gas turbine engine.

An embodiment of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:—

FIG. 1 is a schematic cross section of a prior blade;

FIG. 2 is a schematic cross section of a blade in accordance with thepresent invention;

FIG. 3 is a schematic end view of a prior blade;

FIG. 4 is a schematic end view of a blade in accordance with the presentinvention; and,

FIG. 5 is a schematic view of a blade mounting.

As indicated above, it is known to attach fan blades to a fan rotor discusing a curved dovetail root which is aligned axially. The fan bladeroot is generally in the form of as indicated a dovetail which locatesin a corresponding reciprocal slot in the fan rotor disc. End surfacesof the root are formed so that they are parallel with the front and rearsurfaces of the fan rotor disc and so orthogonal to the enginerotational axis. Such orthogonal presentation of the ends of the rootsections for the blades creates sharp and angular portions which asindicated may concentrate imparted load upon impact with a fan casingshould the blade become detached. Such potentially heavier impact forcesrequire a thicker casing which in turn adds significantly to overallengine weight and therefore reduces efficiency.

FIG. 1 provides an illustration of a prior blade profile cross section.Thus, the blade 1 has a root 2 which incorporates angular corners 3 onthe pressure side of an aerofoil 4. These acute corners as indicatedpreviously will act through the potentially narrow impact zone of theacute point 5 of each corner 3 to impart relatively high impact loads.Such impact loads necessitate thicker and more robust casing profilesand therefore add to overall weight.

It would be appreciated that the corners 3 are created by desire to haveend faces 6 of the root 2 which are orthogonal to the axis of rotationfor an engine incorporating the blade 1 in a blade assembly. It will beunderstood that the root end surfaces 6 will generally be continuous andaligned with front and rear faces of a fan rotor disc as will bedescribed later.

It is reducing the effects of these corners 3 in terms of theirpotential for detrimental impact and penetration of a fan casing whichthe present blade and blade mounting addresses.

FIG. 2 illustrates a blade mounting arrangement in accordance with thepresent invention. A blade 21 with a similar aerofoil 24 to thatdescribed with regard to FIG. 1 is provided. Thus, the aerofoil 24extends from a root 22 which is utilised to secure the blade in amounting arrangement comprising a number of aerofoil blades 24 securedto a fan rotor disc. As can be seen the root 22 now includes endsurfaces 26 which are substantially perpendicular to an axis ofcurvature 27 for the blade 21 or at least turned towards thatorientation. Thus, the ends 26 of the root 22 as indicated previouslyhave a dovetail shaping and are relatively square ended so that thecorners 23 of the roots 22 are less acute and of a smaller dimension.Thus, the impact forces should these corners 23 strike upon a fan casingas a result of failure of the blade 21 will be less severe. By suchangling of the end faces 26 there is a reduction of the severity of theangle 23 in comparison with angle 3 in lever 1.

It will be appreciated that turning of the faces 26 between theorthogonal plane depicted in FIG. 1 toward the perpendicular orientationrelative to the axis of curvature 27 in FIG. 2 will reduce the angularnature of the corner 23. In such circumstances the faces 26 may bepresented at orientations other than perpendicular to the axis 27 inorder to achieve a reduction in the angular nature of the corners 23 butit has been found that perpendicular orientation provides best results.In such circumstances typically, and this will depend upon the severityof the axis of curvature 27, the faces 26 will be at an angle 28 in theorder of 25 degrees to the orthogonal plane, that is to say the plane 29perpendicular to the axis of rotation for an engine incorporating theblades 21 and typically consistent with the front and rear surfaces of amounting rotor disc in which the blades are secured through the roots22.

Previously as shown in FIG. 1 the end face 6 of the root 2 willgenerally be in a plane 9 which extends orthogonally upwards from theend edges 10 of the blade 1. The end plane 29 a in FIG. 2 is consistentwith the plane 9 depicted in FIG. 1 that is to say orthogonal to the endedges 20 of the blade aerofoil 24. In such circumstances due to theangle of presentation of the end faces 26 the axial length of the blade21 is increased. Thus, it may be necessary to increase the axial lengthof the fan rotor disc rim in order to accommodate the increased axiallength of the blade 21 due to the presentation of the faces 26.Alternatively, a chamfer portion of the root 22 which extends beyond theplane 29 a may be removed in order to truncate the root 22 in thissection and so ensure that the blade 21 remains within the normal axialprofile length or the slot of the rotor disc mounting but in suchcircumstances it will be appreciated that the length of engagementbetween the root 22 and the slot in the rotor disc may be reducedcausing a reduction in mounting strength which may be unacceptable in ahigh speed rotating device.

FIGS. 3 and 4 respectively illustrate schematically end views of a priorblade (FIG. 3) and a present blade (FIG. 4). Thus, as can be seen inFIG. 3 a blade 31 again incorporates an aerofoil 34 extending from aroot 32. As shall be described later the root 32 is of a dovetail natureand is slid into a reciprocally shaped slot in a rotor mounting disc inorder to secure the blade 31 in use. In the prior blade based on FIG. 3it will be noted that the root 32 at a front edge 35 is generally flatand as described above creates the angular corner 35 for the blade 31which may cause disproportionate impact damage with a fan casing shouldthe blade 31 fail and disintegrate. It is the severity of the angle at35 which results in acute corners with sharp points (5 in FIG. 1)causing high impact loads.

In FIG. 4 a blade 41 in accordance with the present invention isillustrated with an end face 36 to the root 42. Thus, the blade 41 aspreviously incorporates an aerofoil blade 44 which develops from theroot 42. As compared to the sharp edge 35 depicted in FIG. 3 it will benoted that there is profiling of the transition between the root 42 andin particular the face 46 to the adjacent portions 43 of the aerofoil44. In these transition portions 43 there is less acute angling of theaerofoil 44 which in combination with the produced angular corners inthe root 42 further limits the potential for sharp point impacts with afan casing which may as indicated cause greater damage.

The actual profile cross section in the transition portions 43 may bechosen in accordance with operation requirements and it will beunderstood is dependant upon operational flow stressing on the aerofoil44′ in use. It will be understood that the aerofoil 44 may be stressedsuch that the transitional profile in portions 43 should not reduce theoverall operational efficiency or fatigue life of the blade 41 in use.

The present blade will generally reduce the potential for the blade rootin particular to cause damage to incident portions of the fan casingwithin which the fan blade of the present mounting arrangement and a fanblade assembly is secured. It will be understood if there is greatercontrol of potential impact damage it is possible to more confidentlyuse thinner fan casing thicknesses giving a reduction in overall weight.

FIG. 5 provides a perspective view of a prior blade mounting 50. Thus, ablade 51 includes an aerofoil 54 and a root portion 52. The root 52 andtherefore the blade 51 are secured in a fan rotor disc 60 through a slot61. As can be seen the root 52 has a dovetail cross section and is slidalong the length of the slot 51 during assembly such that an end face 56is generally consistent with end surfaces 62 of the fan disc 60 aboutthe slot 61. The root 52 is secured in the slot 51 through a slide endretainer assembly 63 to ensure appropriate presentation of the blade 51in use. The fan rotor disc 61 incorporates annulus filler fixings forlocation of filler mountings between the blades 51 in use.

As can be seen the edges of the root 52 create angular corners (3 inFIG. 1) and it is the potential for these angular corners to impingeupon fan casings which is avoided by the present configuration for theroot (22 in FIG. 2) and the transition to the blade overall.

The present blade and blade mounting arrangement achieves an overallreduction in weight by reducing the necessary thickness of fan casing toensure that there is no penetration of that casing if the blade shouldfail. This advantage is achieved through altering the angle of the endsof the root portion along with additional profiling of the bladeadjacent of these root ends. In short, by reducing the acuteness of thepoints the impact load area of any fragments from blade failure isbroadened and therefore the impact force spread over a great area of thefan casing.

Alternations and modifications to the present blade and blade mountingwill be appreciated by those skilled in the art and as indicated abovein particular with regards to the actual variation in the root end faceangle to achieve best effect in terms of maintaining fan disc axialwidth in the direction of critical length whilst reducing as indicatedthe angular nature of the corners for the root.

1. A blade for a gas turbine engine, the blade rotatable in use about anaxis of rotation, the blade comprising a root for securing the blade,the root curved for alignment in use with a mounting slot, characterisedin that an end of the root is substantially perpendicular to the axis ofcurvature of the root.
 2. A blade as claimed in claim 1, in which bothends of the root are substantially perpendicular to the axis ofcurvature of the root.
 3. A blade for a gas turbine engine, the bladerotatable in use about an axis of rotation, the blade comprising a rootfor securing the blade, the root curved for alignment in use with amounting slot, characterised in that an end of the root makes an angleof substantially 25 degrees to the plane perpendicular to the axis ofrotation.
 4. A blade as claimed in claim 3, in which both ends of theroot make an angle of substantially 25 degrees to the planeperpendicular to the axis of rotation.
 5. A blade as claimed in claim 1,in which the blade comprises an aerofoil which extends from the root anda portion of the aerofoil adjacent to the end of the root is rounded soas to provide a smooth transition between the aerofoil and the end ofthe root.
 6. A blade as claimed in claim 1, in which at least one end ofthe root is chamfered or truncated, so as to reduce the axial length ofthe root.
 7. A gas turbine engine incorporating a blade as claimed inclaim
 1. 8. A gas turbine engine incorporating a blade as claimed inclaim
 2. 9. A gas turbine engine incorporating a blade as claimed inclaim
 3. 10. A gas turbine engine incorporating a blade as claimed inclaim
 4. 11. A gas turbine engine incorporating a blade as claimed inclaim
 5. 12. A gas turbine engine incorporating a blade as claimed inclaim 6.